Helical springs are used in a wide variety of applications in part due to their simple configuration. Generally, helical springs have an outer diameter sized to fit within a cylindrical bore and/or an inside diameter sized to fit over a rod. Often, the end coils of the spring are ground or otherwise tapered to be in a plane perpendicular or square to the cylindrical axis of the spring so that the reaction of the spring to forces parallel to the cylindrical axis which are applied to the spring includes a minimal amount of radial (side) thrust. Tapering of the ends also reduces the solid height of the spring, i.e., the overall length of the spring in the fully compressed state, so that the spring requires a minimal amount of space in the device of which it forms a part. Helical springs are often used in critical applications, such as medical devices, sensitive instrumentation, fluid power control valves and aerospace equipment.
Helical compression springs are often constructed of a high-carbon spring steel since that metal has high strength and therefore provides high load bearing capability and is inexpensive and readily available. Other suitable metal materials are also used. To date, a variety of metal alloys have been suggested to meet secondary requirements (as detailed in the following paragraph) with varying degrees of success.
However, there are applications for helical springs in which a certain load bearing capability is required, normally provided by spring steel, but for which spring steel is not suitable. For example, spring steel is not suitable for applications that require that the spring be made from a material having properties such as resistance to chemical corrosion, imperviousness to magnetic fields, retention of properties at elevated temperatures, being lightweight, and low thermal and electrical conductivity, among others.
Additionally, many products that use helical springs are made of environmentally friendly materials and are generally recyclable except for the small but critical metallic helical spring. Such products must therefore be disassembled prior to recycling to remove the metal spring from the recyclable components. Such disassembly is expensive to the point that recycling can become cost prohibitive.
It is seen from the foregoing that there is a need for a helical spring made of a material which provides good strength characteristics such as load bearing capability and high strength to weight ratio, and which at the same time is recyclable, highly resistant to corrosion, lightweight, non-magnetic, and has low electrical conductivity and low thermal conductivity. Helical springs formed of plastic material have been suggested for use in applications that require one or more of the properties mentioned above, e.g. resistance to corrosion. However, plastic materials have a relatively low strength as compared to spring steel and traditional spring designs using plastic will generally not provide sufficient load bearing capability or strength to weight ratio for most applications.
Furthermore, while it is desirable to utilize injection molding in the manufacture of plastic products due to its relative economy and high degree of accuracy in forming plastic parts, it is difficult to create practical mold designs for the manufacture of helical springs from plastic material. Conventional mold tool designs have four relatively similar mold sections that come together to form the mold cavity equally for each circular quadrant of the spring. Molten plastic material is injected into the mold cavity formed by the mold sections through a center or core pin around which the four mold sections are situated. After cooling, the mold sections pull apart and the spring is released from the mold. However, this conventional method of tool design cannot achieve the smooth helical shape desired for the part. The main problem is undercuts inherent in a helix which impede the withdrawal of the multi-section mold from the helical spring after the molding operation resulting in kinks every quarter turn at the mating surfaces of the mold sections. It is also difficult to achieve a uniformly smooth surface for the molded product due to the presence of knit lines from material flows during an injection molding process. These kinks and knit lines act as stress points at which breakage of the spring may occur over extended use.